If the decision is negative, you can bring an action before the administrative court. You have a few days in which do so, as soon as the decision has been delivered to you. The notice of the decision you receive will tell you where and when to bring the action. You can apply for legal aid from the court.
If your asylum application is rejected in a way which cannot be challenged, your right to asylum or refugee status will not be recognised. You must then leave Germany by the date specified to you. If you simply allow this deadline to pass, you will be forced to return to your home country (deportation). You must bear the costs of your deportation.
The following link provides information, contact persons and offers of assistance should you decide to voluntarily return to your home country: www.bamf.de/EN/Rueckkehr/rueckkehr-node.html